Ships, specially cargo ships, are generally designed considering to include weight of loaded cargos, etc., therefore, a ship in a state of loaded with no or few cargos takes in sea water, etc. at a port before departure to keep balance of the ship for the necessity of securing the propeller immersion depth and navigational safety, etc. during no cargo. The water used as ballast is called ships' ballast water. The ships' ballast water is sea water, etc. loaded to a ballast tank at a port when departing the port with no cargo, while the ships' ballast water is discharged when loading cargos at a port.
When filling and discharging ships' ballast water to and from ships moving between loading ports and unloading ports of different environments, there is a concern of adversely affecting ecosystems of coasts by difference of microorganisms included in the ships' ballast water between the loading port and unloading port. Therefore, at the international convention for the management of ships' ballast water of ships, the international treaty for the Control and Management of Ships' Ballast Water and Sediments of ships was adopted in February of 2004 and a treatment of ships' ballast water became obliged.
The standard of ships' ballast water treatment established by the International Maritime Organization (IMO) is that the number of living organisms of 50 μm or greater (mainly zooplankton) included in ships' ballast water to be discharged from ships is less than 10 per 1 m3, the number of living organisms of 10 μm or greater but smaller than 50 μm (mainly phytoplankton) is less than 10 in 1 ml, the number of cholera is less than 1 cfu per 100 ml, the number of Escherichia coli is less than 250 cfu per 100 ml, and the number of Intestinal Enterococci is less than 100 cfu per 100 ml.
To satisfy the treatment standard of ballast water as above, as a ballast water treatment system, systems combining a storage device or a shipboard generation device of an active substance of, for example, chlorine-based or oxygen-based (bactericidal agent), a device for adding the active substance to ballast water, and a neutralizer adding device for neutralizing the active substance remaining in ballast water when discharging the ballast water are often adopted.
Such a ballast water treatment system has to be operated reliably by confirming that the ballast water treatment system operates normally during staying at a port, and it is also necessary that a resupply of chemicals and monitoring of an operating state (maintenance check) are done after adjusting the state according to the circumstances of the port in some cases. Particularly, since active substances as a bactericidal agent to be used in the ballast water treatment system are often hazardous to human body, ships and the environment, the maintenance check and other operations of the ballast water treatment system have to be done after a safety check of at least a normal level in a state of wearing a protective equipment, therefore, it takes a long time. Furthermore, it is necessary to make sure that water to be discharged has a safe water quality of not negatively affecting the environment.